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                <h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>在重看数据库的时候，想起来之前数据库查询一个实验，在此把数据库的查询语句汇总一下.</p>
<p><img src="https://cdn.jsdelivr.net/gh/1471748102/cdn@1.8.0/xiaokonglong/20200102193241.jpg" alt=""></p>
<h2 id="首先建立三个表"><a href="#首先建立三个表" class="headerlink" title="首先建立三个表"></a>首先建立三个表</h2><p>学生表：Student(Sno,Sname,Ssex,Sage,Sdept)</p>
<p>课程表：Course(Cno,Cname,Cpno,Ccredit)，其中Cpno为外码</p>
<p>学生选课表：SC(Sno,Cno,Grade)，其中Sno为外码，Cno为外码</p>
<pre class=" language-sql"><code class="language-sql">创建stu数据库
<span class="token keyword">create</span> <span class="token keyword">database</span> stu
创建三个表
<span class="token keyword">create</span> <span class="token keyword">table</span> Student
<span class="token punctuation">(</span>
Sno char<span class="token punctuation">(</span><span class="token number">12</span><span class="token punctuation">)</span> <span class="token keyword">primary</span> <span class="token keyword">key</span><span class="token punctuation">,</span>
Sname  char<span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span> <span class="token keyword">unique</span><span class="token punctuation">,</span>
Ssex char<span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
Sage <span class="token keyword">smallint</span><span class="token punctuation">,</span>
Sdept char<span class="token punctuation">(</span><span class="token number">20</span><span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">create</span> <span class="token keyword">table</span> Course
<span class="token punctuation">(</span>
Cno char<span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> <span class="token keyword">primary</span> <span class="token keyword">key</span><span class="token punctuation">,</span>
Cname char<span class="token punctuation">(</span><span class="token number">40</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
Cpno char<span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
Ccredit <span class="token keyword">smallint</span><span class="token punctuation">,</span>
<span class="token keyword">foreign</span> <span class="token keyword">key</span><span class="token punctuation">(</span>Cpno<span class="token punctuation">)</span><span class="token keyword">references</span> Course<span class="token punctuation">(</span>Cno<span class="token punctuation">)</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">create</span> <span class="token keyword">table</span> Sc
<span class="token punctuation">(</span>
Sno char<span class="token punctuation">(</span><span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
Cno char<span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
Grade <span class="token keyword">smallint</span><span class="token punctuation">,</span>
<span class="token keyword">primary</span> <span class="token keyword">key</span><span class="token punctuation">(</span>Sno<span class="token punctuation">,</span>Cno<span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">FOREIGN</span> <span class="token keyword">key</span><span class="token punctuation">(</span>Sno<span class="token punctuation">)</span><span class="token keyword">REFERENCES</span> Student<span class="token punctuation">(</span>Sno<span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">FOREIGN</span> <span class="token keyword">key</span><span class="token punctuation">(</span>Cno<span class="token punctuation">)</span><span class="token keyword">REFERENCES</span> Course<span class="token punctuation">(</span>Cno<span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<p>插入数据后进行操作</p>
<h2 id="多表查询和单表查询"><a href="#多表查询和单表查询" class="headerlink" title="多表查询和单表查询"></a>多表查询和单表查询</h2><h3 id="单表查询"><a href="#单表查询" class="headerlink" title="单表查询"></a>单表查询</h3><ul>
<li>查询计算机系学生的信息</li>
</ul>
<p><code>select Sno,Sname,Ssex,Ssex,Sage,Sdept from Studentx where Sdept='CZ'</code></p>
<ul>
<li>查询全体学生的人数</li>
</ul>
<p><code>select COUNT(*) as avg from Studentx</code></p>
<ul>
<li>查询全体男生人数和女生人数</li>
</ul>
<p><code>select COUNT(*) as 男 from Studentx where Ssex='男'</code></p>
<p><code>select COUNT(*) as 女 from Studentx where Ssex='女'</code></p>
<ul>
<li>查询每个系中的男生人数并按人数的降序排列</li>
</ul>
<p><code>select Sdept,COUNT(Ssex) from Studentx group by Sdept,Ssex having Ssex='男' order by COUNT(Ssex) desc</code></p>
<ul>
<li>查询名字中带“阳”的学生信息</li>
</ul>
<p><code>select * from Studentx where Sname LIKE '%阳%'</code></p>
<ul>
<li>求被选修的各门课程的平均成绩和选修该课程的人数</li>
</ul>
<p><code>select Cno,COUNT(Sno),AVG(Grade) from scx group by Cno</code></p>
<ul>
<li>查找选修课程超过2门且成绩都在80分以上的学生的学号</li>
</ul>
<p><code>select Sno from Scx group by Sno having COUNT(Cno)&gt;2 and MIN(Grade)&gt;80</code></p>
<h3 id="多表查询"><a href="#多表查询" class="headerlink" title="多表查询"></a>多表查询</h3><ul>
<li>查询未选课的学生情况包含学生学号、姓名和所在系</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Sno<span class="token punctuation">,</span>Sname<span class="token punctuation">,</span>Sdept
<span class="token keyword">from</span> Studentx
<span class="token keyword">where</span> Studentx<span class="token punctuation">.</span>Sno <span class="token operator">NOT</span> <span class="token operator">IN</span><span class="token punctuation">(</span>
<span class="token keyword">select</span> Sno
<span class="token keyword">from</span> Scx
<span class="token punctuation">)</span><span class="token punctuation">;</span></code></pre>
<ul>
<li>查询已选课学生的选课情况包含学生学号、姓名、所选课程、成绩</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Studentx<span class="token punctuation">.</span>Sno<span class="token punctuation">,</span>Sname<span class="token punctuation">,</span>Cname<span class="token punctuation">,</span>Grade
<span class="token keyword">from</span> Studentx<span class="token punctuation">,</span>Scx<span class="token punctuation">,</span>Coursex
<span class="token keyword">where</span> Studentx<span class="token punctuation">.</span>Sno<span class="token operator">=</span>Scx<span class="token punctuation">.</span>Sno <span class="token operator">AND</span> Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Coursex<span class="token punctuation">.</span>Cno</code></pre>
<ul>
<li>查询所有学生的选课情况包含学生学号、姓名、课程号、课程名、成绩；（包含选课和未选课所有学生）</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Studentx<span class="token punctuation">.</span>Sno<span class="token punctuation">,</span>Sname<span class="token punctuation">,</span>Coursex<span class="token punctuation">.</span>Cname<span class="token punctuation">,</span>Grade<span class="token punctuation">,</span>Ccredit 
<span class="token keyword">from</span> Studentx
<span class="token keyword">left</span> <span class="token keyword">join</span> Scx 
<span class="token keyword">on</span> Studentx<span class="token punctuation">.</span>Sno<span class="token operator">=</span>Scx<span class="token punctuation">.</span>Sno 
<span class="token keyword">left</span> <span class="token keyword">join</span> Coursex 
<span class="token keyword">on</span> Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Coursex<span class="token punctuation">.</span>Cno<span class="token punctuation">;</span></code></pre>
<ul>
<li>查询没有学生选的课程信息包含课程号、课程名</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Cno<span class="token punctuation">,</span>Cname<span class="token punctuation">,</span>Cpno<span class="token punctuation">,</span>Ccredit 
<span class="token keyword">from</span> Coursex 
<span class="token keyword">where</span> Coursex<span class="token punctuation">.</span>Cno <span class="token operator">NOT</span> <span class="token operator">IN</span><span class="token punctuation">(</span> 
<span class="token keyword">select</span> Cno 
<span class="token keyword">from</span> Scx 
<span class="token punctuation">)</span></code></pre>
<ul>
<li>查询已被选择课程的课程信息包含课程号、课程名、学生学号、姓名、成绩</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Cname<span class="token punctuation">,</span>Sname<span class="token punctuation">,</span>Studentx<span class="token punctuation">.</span>Sno<span class="token punctuation">,</span>Grade<span class="token punctuation">,</span>Coursex<span class="token punctuation">.</span>Cno
<span class="token keyword">from</span> Studentx<span class="token punctuation">,</span>Scx<span class="token punctuation">,</span>Coursex
whereStudentx<span class="token punctuation">.</span>Sno<span class="token operator">=</span>Scx<span class="token punctuation">.</span>Sno    <span class="token operator">AND</span>    Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Coursex<span class="token punctuation">.</span>Cno </code></pre>
<ul>
<li>查询所有课程的选课情况包含课程号、课程名、学生学号、姓名、成绩；（包含没有学生选课的课程）</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Coursex<span class="token punctuation">.</span>Cname<span class="token punctuation">,</span>Coursex<span class="token punctuation">.</span>Cno<span class="token punctuation">,</span>Studentx<span class="token punctuation">.</span>Sname<span class="token punctuation">,</span>Studentx<span class="token punctuation">.</span>Sno<span class="token punctuation">,</span>Grade
<span class="token keyword">from</span> Coursex
<span class="token keyword">left</span> <span class="token keyword">join</span> Scx 
<span class="token keyword">on</span> Coursex<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Scx<span class="token punctuation">.</span>Cno 
<span class="token keyword">left</span> <span class="token keyword">join</span> Studentx 
<span class="token keyword">on</span> Scx<span class="token punctuation">.</span>Sno<span class="token operator">=</span>Studentx<span class="token punctuation">.</span>Sno<span class="token punctuation">;</span>
</code></pre>
<ul>
<li>查询每个学生选课的总学分，并按总学分的降序排列</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Sno<span class="token punctuation">,</span><span class="token function">Sum</span><span class="token punctuation">(</span>Ccredit<span class="token punctuation">)</span>
<span class="token keyword">from</span> Coursex<span class="token punctuation">,</span>Scx
<span class="token keyword">where</span> Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Coursex<span class="token punctuation">.</span>Cno
<span class="token keyword">group</span> <span class="token keyword">by</span> Sno
<span class="token keyword">order</span> <span class="token keyword">by</span> <span class="token function">sum</span><span class="token punctuation">(</span>Ccredit<span class="token punctuation">)</span> <span class="token keyword">desc</span></code></pre>
<ul>
<li>查询选修了数据库课程且成绩在60分以下的学生的学号、姓名、课程名及成绩</li>
</ul>
<pre class=" language-sql"><code class="language-sql"><span class="token keyword">select</span> Scx<span class="token punctuation">.</span>Sno<span class="token punctuation">,</span>Sname<span class="token punctuation">,</span>Grade
<span class="token keyword">from</span> Scx<span class="token punctuation">,</span>Studentx<span class="token punctuation">,</span>Coursex
<span class="token keyword">where</span> Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Studentx<span class="token punctuation">.</span>Sno <span class="token operator">and</span> Scx<span class="token punctuation">.</span>Cno<span class="token operator">=</span>Coursex<span class="token punctuation">.</span>Cno <span class="token operator">and</span> Cname<span class="token operator">=</span><span class="token string">'数据库'</span> <span class="token operator">and</span> grade<span class="token operator">&lt;</span><span class="token number">60</span></code></pre>
<h2 id="结语"><a href="#结语" class="headerlink" title="结语"></a>结语</h2><p>好多语句已经想不起来了，还是要多使用才能记住.</p>
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